1. Field of the Invention
The present invention relates to an optical system suitable for binoculars, and optical devices such as digital cameras, instant cameras and the like.
2. Description of the Related Art
Conventionally, optical glass or optical resin has been used as material for forming the optical elements used in the optical systems of various optical devices. Although optical glass utilizes many materials to combine refractive index and dispersion, the cost of these materials is disadvantageously high and they are not easily molded to shape. In recent years, optical elements in optical systems often have come to be designed with aspherical surfaces to achieve aberration correction and the like. The additional process of forming optical elements with aspherical surfaces when optical glass is used is difficult and expensive.
Optical resin, on the other hand, has the advantages of being suitable for mass production, having a light weight, and being readily formed into elements having aspherical surfaces. Conventional examples of optical resins include polycarbonate (PC), polymethylmethacrylate (PMMA) and the like.
Improvement of optical characteristics and compactness of optical system is limited by the use of conventional optical resins. Particularly when attempting to reduce color aberration in an optical system, optical elements (such as negative optical power lens elements having typically small Abbe number and positive optical power lens element typically having large Abbe numbers must be used), polycarbonate (PC) is used as the material for negative lens elements, and polymethylmethacrylate (PMMA) is used as material for positive lens elements. The combination of these materials, however, is inadequate to reduce the number of lens elements in making the optical system more compact, such that several optical resin lens elements having a high or low Abbe number are necessary.
Optical resins which are usable as materials for optical elements must satisfy conditions in addition to Abbe number, such as high transparency in the visible region, excellent formability, small birefringence, excellent environmental stability and the like. Therefore, even though a resin has excellent optical characteristics such as refractive index and Abbe number, it cannot be used as an optical resin unless said resin satisfies these other required characteristics.